1. Field of the Invention
The present invention relates to a technology for eliminating interference signals from another search/detection apparatus and stably identifying a target to be searched for by the relevant search/detection apparatus in an environment where there are a plurality of search/detection apparatuses for searching for targets and detecting the number, locations, and the like of the targets.
2. Description of the Related Art
Most pieces of conventional radar equipment used on vehicles or the like adopt a frequency modulated continuous wave (FMCW) as a probe signal because of its simple configuration.
FIG. 1 shows an example of the configuration of such vehicle-mounted FMCW radar equipment. The FMCW radar equipment 12 shown in FIG. 1 is mounted on a vehicle D, and comprises a receiving antenna 31, a receiving unit (RX) 32, an amplification unit 33, a fast Fourier transformation unit (FFT) 34, a transmitting antenna 35, a transmitting unit (TX) 36, and an oscillation unit 37.
The receiving unit 32 comprises a low-noise amplifier 41 and a mixer 42. The amplification unit 33 comprises an intermediate frequency amplifier (IFA) 43 and a low-pass filter (LPF) 44. The transmitting unit 36 comprises a high-power amplifier 46, a branch unit (HYB) 45, and a radio frequency oscillator (RF-OSC) 47. The oscillation unit 37 comprises a base-band oscillator (BB-OSC) 48.
The base-band oscillator 48 generates, for example, a triangle wave from a clock signal CLK and outputs it to the transmitting unit 36. The radio frequency oscillator 47 of the transmitting unit 36 is a voltage-controlled oscillator (VCO) or the like and generates a transmitting signal frequency-modulated by the triangle wave. The branch unit 45 outputs the transmitting signal to the high-power amplifier 46 and the mixer 42. The transmitting antenna 35 transmits a transmitting wave amplified by the high-power amplifier 46. A signal transmitted from the transmitting antenna 35 is reflected by another vehicle U, which is its target and is received by the receiving antenna 31.
The low-noise amplifier 41 amplifies a received signal. The mixer 42 generates a beat signal by mixing the received signal and a part of the transmitting signal. The intermediate frequency amplifier 43 amplifies the beat signal. The low-pass filter 44 eliminates a high-frequency component from the amplified beat signal and generates a base-band signal. The fast Fourier transformation unit 34 performs the fast Fourier transformation of the base-band signal and outputs information about a distance r up to the vehicle U, the relative speed v of the vehicle U, and the like.
The following patent reference 1 relates to a system for avoiding interference in microwave radar equipment using a Gunn diode as an oscillation source.
Patent reference 1: Japanese Patent Application Publication No. 06-160512
The above-described FMCW radar equipment has the following problems.
An autonomous search/detection apparatus such as radar equipment must search for a target and measure specifications, such as its distance and the like, depending only on a signal originated by the relevant apparatus. Therefore, if there is another piece of similar equipment in an observation space, such as an oncoming vehicle, an interference component due to another probe signal mixes into the observation band of the relevant equipment to make it difficult to identify the target.
For example, the vehicle U in FIG. 1 is an oncoming vehicle, a forward running vehicle or the like. If the vehicle U mounts the same radar equipment 11 as the FMCW radar equipment 12, the receiving antenna 31 receives a signal transmitted by the transmitting antenna 21 of the radar equipment 11 as well as a probe signal from the radar equipment 12 which is reflected by the vehicle U. Therefore, an interference component due to the transmitting signal from the vehicle U mixes into the base-band signal.
If information about another probe signal can be obtained by some means, it is not always impossible to eliminate the interference component. However, in a situation where various forms of equipment are mixed, the equipment configuration becomes complex and its signal processing cost increases. In the application of vehicle-mounted radar equipment that requires a high-speed response under the control of a low-performance central processing unit (CPU) or the like, it becomes a fatal disadvantage.
Although the search/detection apparatus measures a distance using the echo of a probe signal originated by the relevant apparatus, its receiving power attenuates in proportion to the fourth power of a distance up to a target while the receiving power from an interference signal source attenuates in proportion to the second power of the distance. In other words, generally the power level of an interference signal is far larger than the power level of a necessary signal. If no countermeasures are taken, there is a high possibility that the necessary signal may be masked by the interference signal.